There has been used a post-processing device that conducts post-processing such as punching processing, stapling processing and sorting processing for sheets on which an image has been formed. For the purpose of making an option sorter connectable with outer equipment of plural types, as a post-processing device, there is proposed a technology to detect, by using an option sorter, the conveying speed for the sheet conveyed from the outer equipment, and thereby to control the conveying speed for the sheet in accordance with the detected conveying speed (for example, see Patent Document 1: Japanese Unexamined Patent Publication TOKKAI No. H03-227848). In many cases, the conveying speed for the sheet is usually adjusted at an entrance portion of a post-processing device as stated above.
However, when the conveying speed of the post-processing device is higher than the conveying speed of the image forming device, a sheet is stretched because of a speed difference when the sheet is conveyed to the post-processing device from the image forming device, and the image surface is rubbed by a conveying roller, which has been a problem. In the case of an image employing toner containing wax, in particular, the conveying roller rubs the sheet under the condition that wax is not solidified sufficiently, because the melting point of the wax is low, which has resulted in easy generation of uneven gloss.
When the conveying speed of the image forming device is different from that of the post-processing device, speed control of acceleration and deceleration is necessary on either one of the image forming device and the post-processing device, and for controlling speed acceleration and deceleration, it is necessary to secure a conveying path with a sufficient length, and it requires a load of providing an extra control mechanism on the image forming device or post-processing device, which has resulted in problems of large-sized apparatuses and complicated controls.
In a case where plural post-processing devices are connected to a single image forming device, neither acceleration nor deceleration is necessary between connected post-processing devices. Therefore, if a conveying path having a sufficient length for controlling acceleration and deceleration is provided on each post-processing device, conveying paths and control mechanism will be wasteful.
A subject of the invention is to prevent uneven gloss caused by acceleration of conveying speed in the case of conveying a sheet from an image forming device to a post-processing device whose conveying speed is higher than that of the image forming device.